Frequently encountered problems, such as loss of efficiency during the operation of steam turbines, are in many instances caused by thermal deformation of the turbine inner cylinder. In many current turbine designs, thermal deformation is transferred to the blade assemblies through the hard connection created by locking devices or other means of firmly attaching the blade assemblies to the inner cylinder. The stationary blade assemblies are subject to thermal deformation caused by the inner cylinder and assume an oval shape, a condition referred to as "ovality." In certain cases, such an out-of-round condition can also be caused by a lack of continuity within the turbine assembly, i.e., from the top half-section of the blade ring to the bottom half-section of the blade ring. Pressure loading and thermal gradients cause the end of each half-section to spread and deflect radially, causing "ovality" and similar clearance problems to those encountered in assemblies connected by locking devices.
When ovality occurs, the clearances in the turbine assembly are altered and steam leakage and decreased efficiency result. The potential for mechanical interference with the rotor is created as well. Presently, to account for the dimensional changes due to thermal deformation, the inner cylinder and the rotating blades are manufactured using larger than optimum clearances, creating a designed-in source of steam leakage and a resulting in lower overall efficiency than would otherwise be achieved.
An additional problem is that the welds joining the stationary blade foils and the inner and outer turbine rings may sometimes crack due to vibrations generated during operation. In present turbine designs, the welded stationary blade assemblies cannot be accurately tuned or otherwise adjusted to avoid matching the operating speed with the natural frequency of the turbine, and harmonic vibration occurs. For example, it has been found that the natural frequency of some nuclear turbine blade assemblies which experienced cracking was at or near the turbine running speed of 30 Hz.
Therefore, it would be desirable to isolate or detach the welded stationary blade assemblies from the turbine inner cylinder to prevent the thermal deformation of the inner cylinder from being transferred to the welded blade assemblies. By providing such detachment, thermal and pressure deformations from the inner cylinder would not be passed to the welded blade assemblies, such that ovality and other deformations which previously resulted in decreased efficiency and increased turbine down time can be prevented.
Additionally, it would be desireable to provide a turbine assembly having stationary turbine blade assemblies which can be optimally controlled in terms of both maintaining appropriate clearances and controlling vibratory frequency. As explained above, present designs require an extra clearance margin to account for ovality and other deformation. However, since the degree of such deformation cannot be exactly calculated, a loss of efficiency, as well as an inability to harmonically balance the turbine results, since clearances remain which are too large, even during operation.